Field of the Invention
The present invention relates to a pentagonal prism for use in a single-lens reflex camera viewfinder and more particularly it relates to such a pentagonal prism which makes use of total reflection from the roof surfaces without metallic reflection treatment thereon.
Generally, a pentagonal prism for use in the viewfinder of a single-lens reflex camera includes 3 reflection surfaces, all of which are coated with metallic films for reflection. The use of aluminum coating for such metallic films results in chemical stability and low costs. Since, however, the reflection efficienty thereof is about 85%, 3 reflections each per reflection surface reduce the quantity of light to about 60%. Silver coating, if applied to the reflection surfaces, produces about 95% of reflection efficiency, and even 3 reflections reduce the quantity of light to only about 85%. In terms of chemical stability and costs, however, it is inferior to aluminum coating. If total reflection is possible on the two surfaces forming the roof surfaces of a pentagonal prism, a metallic film for reflection is required only for one surface, which will result in about 85% of total transmission factor even when aluminum is used for the metallic film, whereby performance as high as that of a prior art pentagonal prism with 3 silver-coated reflection surfaces can be obtained. Aluminum coating on one surface alone is greatly advantageous in costs. From this point of view, there has been proposed a pentagonal prism making use of total reflection on the roof surfaces in Japanese Patent Laid-Open Publicaton No. Sho 49-64425.
Total reflection on the roof surfaces of a pentagonal prism requires an angle formed by a roof ridge and a light incident (bottom) surface, i.e., angle .alpha. shown in FIG. 1 to be larger than an angle determined by the refractive index of glass used for the prism. However, large angle .alpha. increases the size of a camera viewfinder, thereby offending against recent demand for a compact design camera. It is desirable therefore to minimize angle .alpha. so that the light of total reflection on the roof surfaces can reach an eye viewing through a camera viewfinder so long as the eye remains at a regular position on the optical axis of the viewfinder. In this case, however, even the slightest shift of the eye causes the conditions of total reflection on the roof surfaces to be unsatisfied in the corners of a viewfinder image field, and an image therein to be dark, whereby a viewfinder image becomes greatly unstable against the shift of an eye. As a solution to such a problem, a prism may be made of glass with a large refractive index. Generally, however, such glass increases the degree of dispersion, thereby causing a viewfinder image to be colored due to chromatic aberration, posing another problem. That is, a pentagonal prism is optically equivalent to a 70 mm to 80 mm thick glass plate with a parallel plane, and chromatic aberration occurs when an object at a limited distance is viewed through such thick glass plate. In addition, the use of total reflection on roof surfaces poses a problem of ghost due to undesired light, as described later.